The present invention relates generally to optical oscillators and amplifiers used in fiber-optics for telecommunications, cable television and other fiber-optics applications. More particularly, the invention relates to optical oscillators and amplifiers based on Raman gain in a fiber that provides for a particularly simple implementation based on intracavity use of periodic transmission filters.
Optical amplifiers are one of the key enabling technologies for exploiting the bandwidth available in optical fibers. For example, optical amplifiers can be used to compensate for loss in fiber-optic transmission. Loss refers to the fact that the signal attenuates as it travels in a fiber due to intrinsic scattering, absorption and other extrinsic effects such as defects. Examples of optical amplifiers include erbium-doped fiber amplifiers (EDFAs) and Raman amplifiers. A key feature of optical amplifiers is that they be low-noise and broadband, thereby permitting wavelength-division-multiplexed (WDM) systems.
There are two main low-loss telecommunications windows in optical fibers at wavelengths of 1.3 and 1.55 microns. EDFAs have become the workhorse of the optical amplifier field, but they only operate in the 1.55 micron window. Raman amplifiers have the advantage that they can operate in both optical communication windows, and, in fact, over the entire transparency window of optical fibers. Also, Raman gain increases system reliability since there is no excess loss in the absence of pump power. Moreover, Raman-based amplifiers are fully compatible with fiber systems since they are all-fiber devices.
Stimulated Raman scattering amplifiers work on an entirely different principle than EDFAs. Stimulated Raman scattering amplifiers are based on nonlinear polarization of the dielectric silica host, whereas EDFAs are based on the doping of glass fibers with rare earth ions. Signal amplification in Raman amplifiers is due to stimulated scattering accompanied by the excitation of molecules into a vibrational state. In contrast, signal amplification in EDFAs is due to stimulated emission accompanied by relaxation of the excited ions to the ground state. Thus Raman amplifiers and erbium-doped amplifiers work on entirely different physical principles.
The nonlinear polarization in Raman amplifiers is third order in electric field strength, resulting in a nonlinear index of refraction and gain that are both proportional to the instantaneous pump intensity. In contrast, the medium polarization is linear in the EDFA. Also, whereas EDFAs have an upper state lifetime of about 10 msec, Raman amplifiers have a virtually instantaneous response.
The theoretical noise-figure contribution from signal-spontaneous beating for Raman amplifiers has been shown to be 3 dB. However, systems tests of Raman amplifiers have uncovered other sources of noise that generally are not important in EDFAs (c.f. A. E. White and S. G. Grubb, xe2x80x9cOptical Fiber Components and Devices,xe2x80x9d Ch. 7 in Optical Fiber Telecommunications IIIB, eds. I. P Kaminow and T. L. Koch, Academic Press, 1997). The first source is the coupling of intensity fluctuations from the pump light to the signal. The fundamental cause of this noise is the lack of a long upper-state lifetime to buffer the Raman gain from fluctuations in the pump intensity. It has been shown that when a counter-propagating amplifier geometry is used, the transit time of the amplifier can be used to average gain fluctuations due to the pump. Second, double Rayleigh can also give significant contributions to the noise figure of Raman amplifiers because of the long lengths of fiber used. However, limiting the fiber lengths used and constructing multistage amplifiers can control the noise figure of the amplifier.
Several Raman laser and amplifier cavity designs exist as prior-art, but they are not very appropriate for broadband amplification of WDM systems. S. G. Grubb and A. J. Stentz (Laser Focus World, pp. 127-134, February 1996; also U.S. Pat. No. 5,323,404) have described a linear cavity that uses a series of gratings to define the end mirrors. However, the bandwidth of the gratings is sufficiently restrictive that the cavity can operate over only about 2 nm, which is inadequate for WDM applications.
As an improvement, Grubb, et al. (U.S. Pat. No. 5,623,508) also describe a ring cavity that uses an intra-cavity isolator to reduce double Rayleigh scattering and uses a counter-propagating pump to avoid pump fluctuations from coupling to the signal channel. The ring cavity design, however, is substantially more complicated, and, since it also employs gratings, it is also narrow band.
Rather than using gratings, Chernikov, et al. (Electronics Letters, Vol. 31, pp. 472-473, March 1995), use wavelength selective couplers in their Raman cavity design. Whereas their original 1995 design uses five couplers, a simpler configuration using only two couplers is described later (Electronics Letters, Vol. 34, pp. 680-681, Apr. 2, 1998). The couplers used are fused fiber couplers that couple over certain Raman orders into a ring cavity while passing other Raman cascade orders onto an end mirror. By using these broader band devices they achieve a bandwidth between 6-10 nm. However, the couplers may be difficult to manufacture, are somewhat inefficient in that they do not completely couple over or pass through any of the Raman orders, and there are no means in the cavity for rejection or dampening of the double Rayleigh scattering.
Broader band designs of Raman cavities have also been disclosed. As described in U.S. Pat. No. 5,778,014, there are several advantages of the Sagnac Raman amplifier and laser designs over those based on gratings or wavelength-selective couplers. First, the Sagnac cavity is a simple, easily manufactured, all-fiber cavity that should reduce the cost of assembly and increase the device reliability. Second, the passive cavity Sagnac interferometer design has a noise dampening property during the cascaded amplification process, thereby leading to improved noise performance and stability. Third, the broadband cavity design and components should lead to a wider gain bandwidth (in excess of 10 nm) for WDM applications. However, since there is no wavelength control within the cavity, changes in pump power may lead to fluctuations in the output wavelength. Also, the Sagnac requires use of polarization controllers, unless the cavity is made of all polarization-maintaining components. Finally, the Sagnac may have a lower efficiency than linear grating-based cavities since the pump light is split along the two directions of the Sagnac.
An alternate cavity based on a circulator loop cavity and the use of chirped fiber gratings has also been described in the above-noted U.S. patent application No. 60/120,408 Feb. 12, 1999. The chirped gratings can also be composed of a series of gratings. The reflection band of each band is slightly shifted in frequency. The circulator loop design permits a strictly counter-propagating pump for the signal wavelength, and the chirped fiber gratings permit wavelength control while still allowing for broadband behavior for each Raman cascade order. Hence, the circulator loop design can be low-noise and broadband at the same time. However, the design requires circulators or isolators that provide a sufficient amount of isolation over several Raman orders. Such broadband devices are not available as yet, although they could potentially be composed of a cascade of circulators or isolators operating at each Raman order.
Consequently, there is a need for a Raman oscillator or amplifier would have the best features of all of these designs. The desired attributes for the cavity include:
high-efficiency and low intracavity loss, such as in the grating-based designs;
low-noise performance by using strictly counter-propagating pump and signals, such as in the grating-based ring designs;
broadband designs, such as the Sagnac Raman cavity;
stable wavelength operation, such as in the circulator loop cavity with chirped gratings; and
the number of components in the cavity are to be minimized to reduce the intracavity loss and to increase the commercial viability of the design.
One example of a transmission filter is a fiber-based lattice device (c.f., D. A. Nolan, W. J. Miller, and R. Irion, xe2x80x9cFiber-based Band Splitter,xe2x80x9d Optical Fiber Conference 1998, Technical Digest, pp. 339-340.; D. A. Nolan, xe2x80x9cFiber-Based Lattice Devices,xe2x80x9d IEEE International Passive Components Workshop, Italy, September 1998). These devices, fabricated using fibers with different propagation constants, are useful for a number of new and growing applications. For instance, fiber-based Mach-Zehnders can be fabricated using two couplers and fibers with different propagation constants. Mach-Zehnders filters have been shown to have sinusoidal pass bands with peaks every 25 nm or 50 nm. These peaks are separated equally in frequency.
Band splitters can be designed through a synthesis of coherent optical delay line circuits. As an example, the addition of a third coupler to the Mach-Zehnder structure enables a fiber-based lattice component capable of splitting bands. By appropriately choosing the coupler values and the magnitude of the optical path length differences between the couplers, wavelength band splitters can be customized. Filters demonstrated include 1480/1550 EDFA pump/gain band filters and 1310/1550 nm band splitters. These filters can be used as WDMs or as in-line filters for spectral shaping.
It is one object of this invention to provide means for implementing a Raman oscillator in a resonator formed in a linear or ring cavity that uses cascaded Raman distributed gain and an intracavity frequency filter. The attributes of the intracavity filter include:
transmission filter;
pass bands periodic in frequency;
frequency separation of the peaks by 13.2 THz, coinciding with the different Raman cascade orders, or some multiple of this frequency separation;
wide passband for each peak.
It is another object of this invention to provide a means for Raman amplification by using the Raman oscillator to either pump in a counter-propagating fashion an existing transmission link or a Raman gain fiber with one or more isolators.
It is another object of this invention to provide means for a discrete Raman amplifier with counter-propagating geometry for the pump and signal by using either a circulator loop cavity or a ring cavity with an intracavity transmission filter.
It is an object of the present invention to provide a Raman oscillator including an intracavity filter and amplifiers utilizing same wherein linear or ring cavity oscillators using distributed Raman gain are combined with an intracavity frequency filter that is periodic in frequency with peaks corresponding to one or more of the Raman cascade orders.
It is another object of the present invention is to provide a Raman oscillator including an intracavity filter and amplifiers utilizing same wherein Raman oscillators are used with in-line transmission filters to pump a discrete or distributed Raman amplifier for transmission links.
It is yet another object of the present invention to provide a Raman oscillator including an intracavity filter and amplifiers utilizing same wherein discrete Raman amplifiers are implemented in a circulator loop cavity or ring cavity by combining Raman gain and the intracavity transmission filters with a strictly counter-propagating geometry for the pump and signal.
In carrying out the above objects and other objects of the present invention, a Raman oscillator having high efficiency due to low intracavity loss is provided. The oscillator includes at least one laser cavity and a distributed gain medium positioned in the at least one cavity. The oscillator further includes a mechanism adapted to be coupled to a pumping mechanism to pump the distributed gain medium at a pumping wavelength to obtain an optical signal wherein distributed gain is provided by Raman amplification over at least one cascade order corresponding to the pumping wavelength. A filter is positioned in the at least one cavity and includes at least one pass band with a transmission peak placed at the at least one cascade order to filter the optical signal to obtain a filtered optical signal having a signal wavelength. Finally, the oscillator includes an output port for outputting the filtered optical signal.
Preferably, the distributed gain medium is a distributed gain fiber having a single spatial mode over the pumping wavelength to the signal wavelength.
Also, preferably, the filter has an all-glass composition and may be a Mach-Zehnder filter, a low-Q etalon, gratings-separated Fabry-Perots or a fused fiber coupler.
The distributed gain is preferably provided by Raman amplification over a plurality of Raman cascade orders including a final Raman order and the filter has pass bands periodic in frequency with transmission peaks placed at the plurality of cascade orders or multiples of the cascade orders. The transmission peaks placed at the cascade orders are separated by approximately 13.2 THz or some multiple of 13.2 THz or some submultiple of 13.2 THz. The transmission peaks may be separated by two cascade orders.
The distributed gain is preferably provided by Raman amplification over a plurality of cascade orders and the filter may include a band-splitting filter that separates at least one cascade order from at least one other cascade order. The band-splitting filter may separate the pumping wavelength from the signal wavelength.
The oscillator may include a mechanism for splitting the pump wavelength from the signal wavelength outside the at least one cavity wherein the mechanism adapted to be coupled and the output port are combined outside the at least one cavity.
The at least one cavity may be a linear cavity having ends and wherein the filter includes a reflective surface which forms one of the ends of the linear cavity.
The filter may include the mechanism adapted to be coupled.
A coupler may be coupled to the mechanism adapted to be coupled and the output port and may have different periodicity wherein the pumping wavelength and the signal wavelength are separated by a number of cascade orders outside the cavity.
At least one grating may be positioned in the cavity for controlling wavelength of the filtered optical signal. A coupler may be coupled to the mechanism adapted to be coupled and the output port and have different periodicity wherein the pumping wavelength and the signal wavelength are separated by a number of cascade orders outside the cavity.
The cavity may be a ring cavity closed on itself and having a circular cavity geometry. The filter may include an inline periodic filter. The ring cavity may have an arm and wherein fine-tuning elements are positioned in the arm of the ring cavity for fine frequency tuning the filtered optical signal. An isolator may be positioned in the ring cavity to make the ring cavity operate unidirectionally.
The filter may include a many-order periodic filter.
At least one tuning element may be positioned in the cavity for fine frequency tuning the filtered optical signal. The at least one tuning element may comprise a grating, a Fabry-Perot interferometer or a dielectric structure.
Further in carrying out the above objects and other objects of the present invention, in a fiberoptic transmission system including at least one transmission link, an optical amplifier is provided. The amplifier includes an amplifier input port for receiving an optical signal and an amplifier distributed gain medium connected to the amplifier input port to amplify the optical signal. A Raman oscillator, as previously provided, pumps the amplifier distributed gain medium at a pumping level sufficiently high so that the optical signal experiences a gain. An amplifier output port outputs the amplified optical signal.
Preferably, the amplifier distributed gain medium is a gain fiber.
Also, preferably, the gain fiber is pumped in a counter-propagating fashion by the Raman oscillator.
The optical amplifier may be a discrete Raman amplifier which includes at least one isolator and wherein the discrete Raman amplifier is positioned within the at least one transmission link.
The optical amplifier may be a distributed amplifier wherein the at least one transmission link includes a fiber of the amplifier so that the distributed amplifier serves as a low-noise preamplifier.
Still further in carrying out the above objects and other objects of the present invention, in a fiberoptic transmission system, a discrete optical amplifier is provided. The optical amplifier includes an amplifier input port for receiving an optical signal and a Raman oscillator, as previously provided, to pump the distributed gain medium in a counter-propagating fashion at a pumping level sufficiently high so that the optical signal experiences a gain and to provide for cascaded Raman wavelength shifting and wherein the output port outputs the filtered and amplified optical signal.
The ring cavity may have an arm and the optical amplifier further includes fine-tuning elements positioned in the arm of the ring cavity for fine frequency tuning the filtered and amplified optical signal. An isolator may be positioned in the ring cavity to make the ring cavity operate unidirectionally.
Yet still further in carrying out the above objects and other objects of the present invention, in a fiberoptic transmission system, another optical amplifier is provided. The optical amplifier includes an amplifier input port for receiving an optical signal. A Raman oscillator, as previously provided, has a pair of interconnected ring cavities. The distributed gain medium is positioned in each of the ring cavities. The Raman oscillator pumps the distributed gain medium so that the optical signal experiences a gain and to provide for cascaded Raman wavelength shifting. Each cascade order is counter-propagated relative to an adjacent cascade a order. The output port of the Raman oscillator outputs the filtered and amplified output signal.
In accordance with the invention, a Raman oscillator is implemented utilizing distributed Raman gain plus a means of introducing the pump and removing the signal output. An intracavity transmission filter is also used that is periodic in frequency with peaks separated by 13.2 THz and with wide passband for each peak. The transmission peaks are tuned to coincide with the Raman cascade orders corresponding to the pump frequency. A linear cavity design has cavity reflectors formed from coated mirrors or Sagnac loops. A circulator loop cavity design uses an optical circulator to close the loop and provide unidirectional oscillation. The ring cavity design typically uses an optical isolator to provide unidirectional oscillation.
In alternate embodiments, more complicated in-line filters are used to minimize the number of intracavity elements and to increase the cavity efficiency. For example, functions of the intracavity transmission filter can be combined with the means of coupling in the pump and coupling out the signal output. Also, more precise control of the cavity wavelength can be achieved by introducing fine frequency tuning elements such as gratings, Fabry-Perot interferometers or dielectric structures or filters. Moreover, for pump and signal output wavelengths separated by an even number of cascade Raman orders, the cavity may be simplified by using a transmission filter with peaks at alternate Raman orders (i.e., separated by 26.4 THz).
The present invention also relates to implementing Raman amplifiers that are discrete or distributed. Using the Raman oscillator with the in-line filter to pump in a counter-propagating fashion a Raman gain fiber surrounded by one or two isolators and placed within a transmission link forms a discrete amplifier. Alternately, using the Raman oscillator with the in-line filter to pump in a counter-propagating fashion the transmission link directly forms a distributed amplifier. This latter configuration can serve as a low-noise preamplifier and permits graceful upgrade of existing systems.
In other embodiments of the discrete Raman amplifier, the signal is coupled into and out of a circulator loop cavity or the ring cavity design. The pump and signal are strictly counter-propagating, while the same fiber is used to provide for cascaded Raman wavelength shifting as well as Raman amplification of the signal. The amplifier loop also contains the in-line transmission filter and perhaps other fine frequency tuning elements.
The current invention provides for a low-loss linear or ring cavity with a minimum number of intracavity elements, thereby increasing the efficiency and commercial viability of the design. The design uses transmission filters with inherently broad passbands, and the cavities are all-fiber configurations with minimum chance for damage from high pump powers. Furthermore, the in-line filters are preferably mechanically tunable with low temperature sensitivity.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.